Light-emitting diodes (LEDs) are likely to become ubiquitous across all manner of lighting, signaling and display applications in modern day life. Applications in liquid crystal display (LCD) backlighting and general lighting are expected to become the mainstream in the coming decade. Currently, LED devices are made from inorganic solid-state compound semiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue), however, using a mixture of the available solid-state compound semiconductors, solid-state LEDs which emit white light are difficult to produce.
Many strategies to emit white light are based upon combining blue, green and red light in such a way as to stimulate the eye such that white light is perceived. This may be done with dichromatic, trichromatic or polychromatic light sources. In the case of LEDs it may be achieved by combining multiple LEDs that emit blue, green and red in the correct intensity ratios or by combining blue or UV-LEDs with appropriate color conversion materials. In this case the color conversion material is placed on top of the solid-state LED whereby the light from the LED (the “primary light”) is absorbed by the color conversion material and then re-emitted at a different frequency (the “secondary light”), i.e. the color conversion materials down convert the primary light to the secondary light. In the case of using LEDs combined with color converter materials there are a number of strategies that may be used such as a dichromatic solution whereby a blue LED is combined with a broad yellow emitting material, or a trichromatic solution whereby a blue LED is combined with broad green/yellow and red emitting converter materials. This may be extended to UV-LEDs by including a further blue emitting converter material to both solutions. Simulated spectra of white of white di-, tri- and quad-chromatic light sources are shown in FIG. 1.
Although it is possible to produce white light by combining the light from individual red, green and blue LEDs the use of white LEDs produced using color conversion materials gives advantages such as lower numbers of LEDs being used and simpler circuitry design. Consequences of this include simpler device fabrication and ultimately lower cost.
There are many known color converter materials including phosphors, semiconductors, dyes and more recently semiconductor QDs. The materials in most prevalent use are phosphors which consist essentially of an inorganic host material doped with an optically active element. Common host materials are nitrides, oxides, oxynitrides, halophosphates, garnets, etc. and among the large amount of host materials available the garnets are of particular importance and within the garnet group yttrium aluminium garnet is a particularly common host material. The optically active dopant is typically a trivalent rare-earth element, oxide or other rare-earth compound, for example europium (Eu), cerium (Ce) and terbium (Tb).
White LEDs made by combining a blue LED with a broad yellow phosphor may be very efficient, however, there are problems such as color control and color rendering due to a lack of tunability of the LEDs and the phosphor. Color control refers to the final color of the LED when the LED light is combined with the emission of the phosphor. This color is inherently limited by the emission spectrum of the phosphor which is not particularly tunable by composition. In order to change the color of the LED a different phosphor material is necessary. Color rendering refers to the ability of the light source to illuminate objects such that the color that appear is rendered correctly or as similarly as it would appear if the object were illuminated with a blackbody radiator of the same color temperature as the LED light source. Again this is limited by the emission spectrum of the phosphor since to date no one phosphor material can emit light such that the spectrum of a black body radiator can be mimicked exactly so usually a combination of phosphors are necessary and typically the color rendering performance is compromised in favour of luminous performance. Typically blue LEDs combined with broad yellow phosphors have a color rendering index (CRI) of less than 75 and only increase to about 85 when combined with an additional red phosphor. By definition a black body radiator with a color temperature the same as the test LED has a CRI of 100. More recently LEDs combining a broad yellow/green phosphor with red QDs has produced CRIs above 90. Achieving high CRIs is made possible by the use of QD color conversion materials because of the inherent tunability which allows the emission wavelength to be matched with the emission of a broad phosphor to produce light with a high CRI value.
There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions in the order of 2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties which may be exploited in many commercial applications such as optical and electronic devices and other applications ranging from biological labeling, photovoltaics, catalysis, biological imaging, LEDs, general space lighting and electroluminescent displays amongst many new and emerging applications.
The most studied of semiconductor materials have been the chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. Reproducible methods for the large scale production of these materials have been developed from “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles, using “wet” chemical procedures.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are at least in part responsible for their unique properties. The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor being, with many materials including semiconductor nanoparticles, that there is a change in the electronic properties of the material with size, moreover, because of quantum confinement effects the band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the “electron and hole”, produced by the absorption of electromagnetic radiation, a photon, with energy greater than the first excitonic transition, are closer together than they would be in the corresponding macrocrystalline material, moreover the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition of the nanoparticle material. Thus, QDs have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Core semiconductor nanoparticles, which consist essentially of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface which may lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds on the inorganic surface of the OD is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material epitaxially on the surface of the core particle, to produce a “core-shell” particle. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. One example is a ZnS shell grown on the surface of a CdSe core. Another approach is to prepare a core-multi shell structure where the “electron-hole” pair is completely confined to a single shell layer consisting essentially of a few mono layers of a specific material such as a quantum dot-quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few mono layers of HgS which is then over grown by a monolayer of CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer. To add further stability to QDs and help to confine the electron-hole pair one of the most common approaches is by epitaxially growing a compositionally graded alloy layer on the core to alleviate strain that could otherwise led to defects. Moreover for a CdSe core in order to improve structural stability and quantum yield, rather growing a shell of ZnS directly on the core a graded alloy layer of Cd1-xZnxSe1-ySy may be used. This has been found to greatly enhance the photoluminescence emission of the QDs.
There have been two different approaches to using QDs as color converting materials in LEDs, direct addition and as remote phosphors.
Rudimentary QD-based light emitting devices based upon the direct addition principle have been made by embedding colloidally produced QDs in an optically clear LED encapsulation medium, typically a silicone or an epoxy, which is then placed in the well of the package over the top of the LED chip. The use of QDs potentially has some significant advantages over the use of the more conventional phosphors, such as the ability to tune the emission wavelength, strong absorption properties and low scattering if the QDs are mono-dispersed.
For the commercial application of QDs in next-generation light emitting devices, the QDs are preferably incorporated into the LED and in to the encapsulating material in such a way so that they remain as fully mono-dispersed as possible and do not suffer significant loss of quantum efficiency. Problems that QDs face in direct addition LEDs include a) photo-oxidation, b) temperature instability, and c) loss of quantum yield with increasing temperature.
Existing methods developed to date to address photo-oxidation are problematic, not least because of the nature of current LED encapsulants which are porous to oxygen and moisture, allowing oxygen to migrate to the surfaces of the QDs, which may lead to photo-oxidation and, as a result, a drop in quantum yield (QY). Furthermore, QDs may agglomerate when formulated into current LED encapsulants thereby reducing the optical performance.
Concerning heat degradation, QDs are stable to temperatures up to known threshold temperatures depending upon the type of QD whereby ligands desorb from the surface and/or reactions with the resin material and air start to occur. In situations whereby ligand loss occurs, if this is ligand loss is irreversible then the QDs will be irreversibly damaged.
Temperature of operation may affect the performance of the QDs because photoluminescence efficiencies decrease with increasing temperatures. Typically the hottest place within the LED package is located at the LED junction. Often the junction temperature may be much hotter than the surrounding package.
Although reasonably efficient QD-based light emitting devices may be fabricated under laboratory conditions building on current published methods and taking into account the three key issues discussed above, there remain significant challenges to develop materials and methods for fabricating QD-based light emitting devices under commercial conditions on an economically viable scale.
With regard to the use of QDs as remote phosphor color converting materials in LEDs, devices have been developed in which the QDs are embedded into an optically clear medium, typically in the form of a sheet or strip. The requirements for the optically clear medium are similar to those for direct addition in that the QDs are preferably fully dispersible in the optically clear medium and suffer little loss of quantum efficiency.
The QDs face similar problems in phosphor sheet materials as in devices based upon the direct addition principle, i.e. photo-oxidation, temperature instability and loss of quantum yield with increasing temperature as discussed above. Furthermore, problems arise from the remote phosphor format itself such as a) light trapping from waveguiding in the sheet type structure reducing performance, b) high material usage and c) lower performance than direct LEDs depending on distance from the LED light source.